Plasma torches are often used for cutting and/or marking operations. In general, plasma torches comprise a gas nozzle and an electrode. A gas is directed through a nozzle toward a workpiece. Some or all the gas is ionized into a plasma arc between the electrode and the workpiece. The arc is used to cut, mark or otherwise operate on the workpiece.
In most plasma torches, a pilot arc is established between the electrode and the nozzle. The pilot arc is then transferred from the electrode to the workpiece for cutting and/or marking. In blow back start torches, for example, current can be passed through the nozzle and electrode when they are spring loaded in contact with each other. When the flow of gas is then initiated or increased, the gas pressure acts in opposition to the spring which forces the electrode and nozzle drawing a pilot arc. The flowing gas acts to blow the pilot arc out the nozzle to make electrical connection from the electrode to the workpiece establishing the main arc. As another example, a high electric potential can be applied to the nozzle and electrode to create a spark, thereby initiating the pilot arc.
As will be appreciated, to use a plasma torch for cutting, the stream of plasma must be repeatedly cycled. More specifically, the stream of plasma must be turned on and off repeatedly in order to mark the workpiece correctly. Furthermore, the restart cycle of the plasma stream must be coordinated or synchronized with movement of the nozzle for the markings to be made where intended. Conventional blow back type torches are relatively slow to cycle on and off. This can be due to a number of factors, including the need to perform a parts-in-place check, the time require to pressurize and depressurize gas lines, and the like. Furthermore, the exact time at which the main arc is established and is extinguished is imprecise.
Some plasma torches (often referred to as “high frequency start torches”) use a high electric potential at high frequency to restart the plasma arc. Although high frequency torches have quicker restart times than blowback torches, the high electric potential, high frequency discharges result in conducted and radiated noise emission that often interferes with nearby electronics. This is especially problematic for the case of marking where the nozzle movement is electronically controlled.
Accordingly, a technique for marking a workpiece with a plasma torch is needed that is quicker than conventional techniques and does not interfere with electronics and/or computers.